A new class of secondary cells, i.e., batteries, are metal hydride secondary cells. These cells eliminate the toxic cadmium negative electrodes of conventional nickel-cadmium cells, substituting therefor a metal hydride negative electrode. Most of the research activity in this field has involved the lanthanum penta-nickel alloy system, the titanium-nickel alloy system, and the iron-titanium alloy system. These alloy systems all suffer from various deficiencies, e.g., low storage capacity, poor thermodynamics, or poor kinetics.
One hydrogen storage alloy system that overcomes these disadvantages is the titanium-vanadium system of hydrogen storage alloys. This system is exemplified by the titanium-vanadium- nickel- zirconium and titanium- vanadium- nickel-zirconium- chromium systems of hydrogen storage alloys and when the term "titanium-vanadium" is used herein with respect to hydrogen storage alloys, such alloys also containing nickel, zirconium, chromium, aluminum, iron, and other additives are encompassed thereby unless the context indicates to the contrary.
Exemplary titanium-vanadium alloys include titanium-vanadium- nickel- aluminum alloys; titanium- vanadium- nickel-zirconium alloys; titanium- vanadium- chromium- nickel alloys; titanium- zirconium- vanadium- nickel- chromium alloys; and titanium- vanadium- manganese- iron alloys. The preferred titanium-vanadium type alloys are those having the stoichiometries:
(1). titanium- vanadium- nickel- aluminum alloys having the formula (TiV.sub.2-x Ni.sub.x).sub.1-y Al.sub.y, where x is from 0.2 to 1.0, and y is from 0 to 0.2;
(2). titanium- vanadium- nickel- zirconium alloys having the formula (TiV.sub.2-x Ni.sub.x).sub.1-y Zr.sub.y, where x is from 0.2 to 1.0, and y is from 0 to 0.2;
One hydrogen storage alloy system that overcomes these disadvantages is the titanium-vanadium system of hydrogen storage alloys. This system is exemplified by the titanium-vanadium- nickel- zirconium and titanium- vanadium- nickel-zirconium- chromium systems of hydrogen storage alloys and when the term "titanium-vanadium" is used herein with respect to hydrogen storage alloys, such alloys also containing nickel, zirconium, chromium, aluminum, iron, and other additives are encompassed thereby unless the context indicates to the contrary.
Exemplary titanium-vanadium alloys include titanium-vanadium- nickel- aluminum alloys; titanium- vanadium- nickel-zirconium alloys; titanium- vanadium- chromium- nickel alloys; titanium- zirconium- vanadium- nickel- chromium alloys; and titanium- vanadium- manganese- iron alloys. The preferred titanium-vanadium type alloys are those having the stoichiometries:
(1). titanium- vanadium- nickel- aluminum alloys having the formula (TiV.sub.2-x Ni.sub.x).sub.1-y Al.sub.y, where x is from 0.2 to 1.0, and y is from 0 to 0.2;
(2). titanium- vanadium- nickel- zirconium alloys having the formula (TiV.sub.2-x Ni.sub.x).sub.1-y Zr.sub.y, where x is from 0.2 to 1.0, and y is from 0 to 0.2; herein by reference. One particularly preferred class of these of these alloys is described with particularity in U.S. Pat. No. 4,728,586 for Enhanced Charge Retention Electrochemical Hydrogen Storage Alloys And An Enhanced Charge Retention Electrochemical Cell, above, and has the formula (Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y).sub.1-z Cr.sub.z, where x is from 0.00 to 1.50, y is from 0.6 to 3.5, and z is an effective amount less then 0.20. The titanium-vanadium hydrogen storage alloys described in the above cited patents typically contain at least about 20 atomic percent vanadium, and sometimes as high as 53 percent vanadium, atomic basis. This high vanadium content is identified with very good thermodynamics, kinetics, and corrosion resistance.
While titanium-vanadium alloys have highly desirable chemical, thermodynamic, and microstructural properties for hydrogen storage, especially electrochemical hydrogen storage, they have not received the research, development, or commercial attention commensurate with their outstanding properties. This is because vanadium alloys have historically been expensive alloys, treated with the commercial respect given precious metal alloys. Indeed, research efforts in the field of electrochemical hydrogen storage alloys have traditionally avoided vanadium alloys due to the high costs associated with the vanadium constituent thereof. Thus, if the highly desirable properties associated with vanadium containing hydrogen storage alloys are to be realized, a need clearly exists for a low cost route to these vanadium rich hydrogen storage alloys.
A major factor contributing to the expense of these alloys is the high cost associated with obtaining metallic vanadium having what has heretofore been considered the requisite purity for incorporation into electrochemical hydrogen storage alloys. This high recovery cost is due to the chemical stability of the refractory vanadium oxide. Processes that work for other refractory oxides do not work with vanadium oxide. For example, the Kroll process is used to recover titanium and zirconium via the reaction system TiO.sub.2 +2 Cl.sub.2 =TiCl.sub.4, TiCl.sub.4 +2 Mg=Ti+2 MgCl.sub.2. This process fails with vanadium, yielding only a lower valence vanadium oxychloride upon magnesium reduction. This lower valence vanadium material is unsuitable for subsequent formation into an electrochemical hydrogen storage alloy.
Likewise, aluminothermic reduction requires a substantial excess of aluminum to completely reduce the vanadium oxide and recover metallic vanadium. Moreover, the use of excess aluminum results in a vanadium-aluminum alloy product containing a substantial amount of metallic aluminum alloyed with the vanadium, typically fifteen percent aluminum (by weight). Aluminum presence to this degree is generally too high to be of use in the preparation of hydrogen storage alloys. Note however, that U.S. Pat. No. 4,551,400 for Hydrogen Storage Materials And Method Of Sizing And The Same For Electrochemical Applications, above, specifically discloses electrochemical hydrogen storage alloys having the formula (TiV.sub.2-x Ni.sub.x).sub.1-y Al.sub.y, where x is from 0.0 to 1.0, and y is from 0.0 to 0.2, the maximum atomic ratio of V to Al is 80:20, and the maximum weight ratio of V to Al is 88:12. While this is below the 85:15 of the commercially available V-Al alloys, it is still high.
Thus, vanadium based electrochemical hydrogen storage alloys have heretofore been commercially limited by the cost of recovering high purity vanadium.